Somewhere Out There

January 31, 2008
NASA, ESA, A. Bolton (Harvard-Smithsonian CfA) SLACS Team

NASA, ESA, A. Bolton (Harvard-Smithsonian CfA) SLACS Team

Black holes, large and small? Bodies so massive they bend light?
Excuse us, but ... what? A few simple explanations may help.

Black holes

A black hole is the result of a massive object like a star collapsing to such a small size that it forms a "singularity" or force of unstoppable gravitational pull. ("Small" is relative in intergalactic terms; black holes can range from a few miles to a few million miles across.) The pull is so strong that nothing that ventures within a certain distance can escape. Everything within that distance—it's called the "event horizon," because events within it are beyond  the horizon of any outside observer—is hidden, so we call the singularity a black hole. It's the great cosmic bathtub drain, a Gravity Motel, so massive that even light checks in but doesn't check out. What happens to that stuff in there? "Unfortunately," says Arlie Petters, professor of mathematics and physics, "the laws of physics break down at this drain hole, so we don't fully understand what is going on there."

Okay, those are the big ones. Now, tiny black holes. What exactly are they?

Nowadays, from what scientists know, only vastly massive objects can collapse and become black holes. But in the hot, crazy environment at the dawn of the universe, objects of all kinds of masses could become black holes. In an Einsteinian universe, the tiny black holes created at the dawn of time would have all evaporated by now, through thermal radiation—in essence, a cooling down via a quantum process called Hawking radiation (after the scientist Stephen Hawking). "Every object has a temperature, including black holes," Petters says. Hawking developed an equation that tells, among other things, how quickly a black hole radiates heat. "Tiny black holes—atomic size or even smaller —in Einstein's theory, radiate heat very fast, so that's why they fizzle out quickly. Black holes in the Randall-Sundrum brane-world model have a different temperature law, making them evaporate more slowly, so such tiny black holes may still exist today."

What is gravitational lensing?

When light passes by a massive object such as a star or even a  planet, the object's gravitational pull is so strong that it actually bends the light passing by it. Think of photons as golf balls and the strong gravitational pull of a large object as a dip in the green. The photons keep going generally where they were going, but their path is altered.

How come light is able to come close enough to a black hole to be bent but not close enough to be pulled in?

It's connected to that event horizon around black holes. There is a region just outside the event horizon called a photon sphere. Light that gets close to the photon sphere without going inside it is bent, and may even loop around the photon sphere many times before continuing on its path, but is not captured. However, light that gets too close to the photon sphere, even just grazing it, may be captured and loop around the sphere forever. "Light that enters the photon sphere at an angle won't be able to get back out," Petters says. "It will get pulled in, never to be seen again.

"The bent light that makes it to Earth will carry clues about the nature of the black hole," he says. "The most energetic beams will be accessible to NASA's new GLAST telescope."